It has long been known that halogenated polymers, such as commercially important polyvinyl chloride (PVC), are subject to deterioration or degradation when exposed to heat and light. Various chemical compounds and compositions have been developed to stabilize these polymers against such deterioration and attendant discoloration. For the most part, these stabilizers have been directed toward stabilization against heat, such as that encountered during processing of the polymer and its fabrication into articles. Among these heat stabilizers, some of the most effective have been the organotin stabilizers. Mono- and dialkyltin mercaptides and their sulfides, mono- and dialkyltin carboxylates, and mixtures of these organotin mercaptides and carboxylates, for example, are currently among the most widely used heat stabilizers.
While organotin stabilizers are excellent heat stabilizers, they provide less than desired weathering and light, particularly ultraviolet light, stabilization. This is evident in articles formed from such organotin-stabilized halogenated polymer compositions. Upon prolonged exposure to light, these articles tend to yellow and decompose with attendant loss in physical properties, e.g., tensile strength, flexibility, and impact resistance, which shortens the useful life of the articles. Haziness, which sometimes accompanies the color changes, is particularly undesirable where clear products are needed. Organotin carboxylates and organotin mercaptide/carboxylates offer better ultraviolet light stability and weathering properties than do organotin mercaptides, although all have generally beer, inadequate for weatherable applications, for example, in articles designed for outdoor use.
With increased use of rigid (unplasticized) halogenated polymer compositions for articles that will be exposed to ambient light and weathering for long periods of time, for example, vinyl housing siding, window frames, window profiles, corrugated roofing, and door stripping, it has become increasingly important to develop a stabilizer that will not only protect the halogenated polymer against heat encountered during processing and fabrication, but will also protect the polymer against the long term effects of light and weathering. Such a stabilizer is particularly needed for clear halogenated polymer compositions which are known to have very poor light stability and weatherability, as they generally do not contain fillers and pigments, such as titanium dioxide, which inhibit light from penetrating into the articles formed therefrom and thus reduce the rate of polymer degradation. However, until the present invention, no single stabilizer composition was completely satisfactory for protecting halogenated polymer compositions, particularly clear halogenated polymer compositions, against heat as well as light and weathering.
Attempts have been made in the past to improve the weathering and light stability of organotin stabilizers. For instance, it has been proposed to combine an organotin stabilizer with certain non-metallic organic compounds known to function as ultraviolet light absorbers, such as benzophenones and benzotriazoles. However, such compounds are typically very expensive, due to the complexities of their manufacture, with the result being that in relatively low cost products, the price of such ultraviolet stabilizers is difficult to bear.
The use of less expensive ultraviolet stabilizers, such as phenyl salicylate compounds, in combination with non-organotin heat stabilizers have also been proposed. For example, it is taught in U.S. Pat. No. 2,824,079 (Fischer) to stabilize flexible (plasticized) polyvinyl chloride compositions against the degrading effects of light in addition to heat by adding thereto small amounts of a phenyl salicylate compound in combination with a bisphenol heat stabilizer. A lead-based heat stabilizer is also suggested for this combination.
U.S. Pat. No. 3,505,383 (Seki et al.) and U.S. Pat. No. 3,600,416 (Seki et al.) teach the use of phenyl salicylate compounds as ligands alone or in combination with other ligands, such as mercaptide or carboxylate ligands, which are directly bound to a tin atom of an organotin stabilizer for improved light stability. However, when phenyl salicylate ligands are employed with mercaptide or carboxylate ligands, the stabilizers so-formed contain a much lower mercaptide or carboxylate content as compared to traditional organotin stabilizers of the same class. It is well known that reducing the mercaptide or carboxylate ligand content tends to adversely affect thermal stability.
U.S. Pat. No. 5,030,676 (Wallen) only generally mentions the use of various ultraviolet stabilizers, inclusive of salicylates, in combination with various heat stabilizers, inclusive of organotin mercaptides and organotin carboxylates and, besides, specifically focuses on opaquely pigmented (TiO.sub.2 /MgO) halogenated polymer compositions. Moreover, attempts by the present inventors to use such a combination to improve weathering and light stability of pigmented (opaque) halogenated polymer compositions have never been successful. When unbonded (free) phenyl salicylates are employed in pigmented halogenated polymer compositions in the presence of organotin stabilizers, it has been found that the salicylates are not functionally active therein and thus do not impart any benefit in terms of light and weathering stability to the pigmented compositions. One skilled in the art would generally expect this stabilizer combination to perform no differently in clear halogenated polymer compositions.
Thus, in spite of the progress that has been made in the industry and the plurality of stabilizer combinations that have been proposed for improving weathering and light stability, organotin mercaptides, their sulfides, organotin carboxylates, and mixtures thereof, are still being used alone in clear halogenated polymer compositions intended for weatherable applications due to the high cost and/or ineffectiveness of the proposed combinations. Much effort is, therefore, still being devoted to finding better stabilizers for clear halogenated polymer compositions, especially for the commercially important clear rigid polyvinyl chloride compositions devised for exterior weatherable applications, which stabilizers are inexpensive and will give desired weathering and light stability in addition to heat stability.
It is, therefore, a primary object of this invention to provide stabilizer compositions comprising synergistic combinations of organotin mercaptides (including their sulfides), organotin sulfides and/or organotin carboxylates and free phenyl salicylates that are not only inexpensive, but that also unexpectedly offer light stability and weatherability in addition to heat stability to clear polyvinyl chloride and other halogenated polymer compositions.